More generally, a breakpoint is a means of acquiring knowledge about a program during its execution. During the interruption, the programmer inspects the test environment (general purpose registers, memory, logs, files, etc.) to find out whether the program is functioning as expected. In practice, a breakpoint consists of one or more conditions that determine when a program's execution should be interrupted.
The most common form of a breakpoint is the one where the program's execution is interrupted right before a programmer-specified instruction is executed. This is often referred to as an instruction breakpoint.
Other kinds of conditions can also be used, such as the reading, writing, or modification of a specific location in an area of memory. This is often referred to as a conditional breakpoint, a data breakpoint, or a watchpoint.
Breakpoints can also be used to interrupt execution at a particular time, upon a keystroke etc.
When a breakpoint is hit, various tools are used to inspect the state of the program or alter it. Stack trace of each thread may be used to see the chain of function calls that led to the paused instruction. A list of watches allows to view the values of selected variables and expressions. There may also be tools to show the contents of registers, loaded program modules and other information.
Many processors include hardware support for breakpoints (typically instruction and data breakpoints). As an example, the x86 instruction set architecture provides hardware support for breakpoints with its x86 debug registers. Such hardware may include limitations, for example not allowing breakpoints on instructions located in branch delay slots. This kind of limitation is imposed by the microarchitecture of the processor and varies from processor to processor.
Without hardware support, debuggers have to implement breakpoints in software. For instruction breakpoints, this is a comparatively simple task of replacing the instruction at the location of the breakpoint by either:
- an instruction that calls the debugger directly (e.g. a system call) or
- an invalid instruction that causes a deliberate program interrupt (that is then intercepted/handled by the debugger)
- an instruction set simulator can implement unconditional or conditional breakpoints, by simply embedding the appropriate condition tests within its own normal program cycle - that also naturally allows non-invasive breakpoints (on read-only programs for instance).
- Interpreted languages can effectively use the same concept as above in their program cycle.
- "Instrumenting" all the source code with additional source statements that issue a function that invoke an internal or external debug subroutine, is yet another common approach. This method increases the binary size and might adversely affect normal memory allocation and exception handlers. "Debug" options exist on some compilers to implement this technique semi-transparently.
Some debuggers allow program variables in memory to be modified before resuming, effectively allowing the introduction of "hand-coded" temporary assignments for test purposes. Similarly, program instructions can often be skipped to determine the effect of changes to the program logic - enabling questions about program execution to be answered in a direct way (i.e. without assumptions or guesswork). In many cases it may be the only practical method of testing obscure "event-driven" error subroutines that rarely, if ever, get executed - without the added risk of leaving temporary source changes.
Implementing data breakpoints in software however, can greatly reduce the performance of the application being debugged - since it is using additional resources on the same processor. However, this is normally acceptable during testing and the amount of information available from the debugger is not restricted by limitations of debug data known to the hardware. For instance, a software implementation can collect logical path data at program/subroutine/instruction level to considerably augment what might be stored by the particular hardware platform for inspection. The instruction set simulation method considerably reduces the compared to the (repeated) instruction replacement method, also reducing cache misses.
Some programming language implementations expose their debugging functions for use by other programs. For example, some FORTRAN dialects have an
AT statement, which was originally intended to act as an instruction breakpoint. Python implements a debugger accessible from a Python program. These facilities can be and are abused to act like the COMEFROM statement.
- IBM OLIVER (CICS interactive test/debug)
- Program animation (Stepping)
- SIMON (Batch Interactive test/debug)